A vehicle control system may include an electrical control unit (ENG ECU) for controlling the driving state of the engine of a vehicle and an electrical control unit (ECT ECU) for controlling an automatic transmission of the vehicle. This vehicle control system uses a dedicated bus through which ENG ECU and ECT ECU are connected to each other because the communication data amount between ENG ECU and ECT ECU is large. Accordingly, the communication load imposed on a common bus to which other electrical control units (ECUs for brake, suspension, air bag, etc.), etc. are connected can be reduced.
An abnormality diagnostic device may be connected to the common bus in the above control system. When the abnormality diagnostic device generates a message concerning an abnormality diagnosis from each ECU, the ENG ECU is required to apply gateway processing to a communication frame for transmitting the message from the ECT ECU so that the communication frame is transmitted from the dedicated bus to the common bus.
Furthermore, the ENG ECU is required to not only apply the gateway processing to the communication frame from the ECT ECU, but also transmit a communication frame representing a message created by itself through the common bus to the abnormality diagnostic device. In this case, if the ENG ECU preferentially transmits its own communication frame corresponding to the message created and then carries out the gateway processing on the communication frame from ECT ECU, the transmission timing of the message from the ECT ECU will be delayed.
The transmission time in the abnormality diagnosis operation is defined by the ISO15031-5 standard or the like. Accordingly, it is necessary to transmit a first transmission frame within 50 ms at the latest from the time when a message transmitting request is made from the abnormality diagnostic device.
However, when priority is given to the transmission processing of the message of the ENG ECU as described above, there may occur a case where it is impossible to transmit the message from the ECT ECU within the time set by the standard in accordance with the length of the message.